The National Stadium, also known as the Bird's Nest, is seen over a man-made lake at the Olympic Green, against the backdrop of the blue sky in Beijing, August 1, 2008. The Beijing sky turned from grey to blue Friday, with the average air pollution index dropping to 28, dubbed as "level 1" or "excellent", local environment authorities said. A pollution reading below 50 is classified as "excellent", between 51 an 100 "fairly good", between 101 and 200 "slightly polluted", between 201 and 300 "poor", and over 301 "hazardous", the Beijing environment bureau said. [Xinhua]